Ground fault detection circuit

ABSTRACT

A controller for a starter/generator used with an aircraft engine, includes means for monitoring field return current in a generator field winding; means for monitoring generator voltage; and microprocessor control means for adjusting generator field current as a function of the field current and voltage and field return current. The field return current and output voltage monitoring functions allow for open field, field integrity, over voltage, field weakening and torque limiting functions to be realized in an integrated programmable system configuration.

This application is continuation of application Ser. No. 08/742,855,filed Nov. 1, 1996, now U.S. Pat. No. 6,107,775.

BACKGROUND OF THE INVENTION

The invention relates to controllers for generators, such as are usedwith aircraft engine starters and generators.

It is well known to use generators as starters for motors and engines.For example, in the aerospace industry, DC generators are commonly usedas engine starters on small aircraft engines such as gas turbineengines. After the engine is started, the generator typically is used asan electrical power source for the aircraft.

Aircraft manufacturers and governmental regulations have, over theyears, gradually increased the demand for more monitoring and controlfunctions to be implemented for these generators, particularly withrespect to providing fast and accurate indications of fault conditionsto avoid catastrophic generator failures and engine damage. However,space and weight considerations can limit the availability and use ofmultiple discrete logic circuits. Discrete control circuits alsoinherently lack central processing control, thus limiting the number ofinterdependent control functions that can be implemented.

Furthermore, various control and monitoring functions heretofore usedwith generator discrete controllers can be inadequate or unsuitable forprotecting components that can be overstressed or for facilitatingacceptable generator operation. For example, linear current limiters fordevices subject to current overloads can exhibit large powerdissipation, and known switch mode current limiters can deliverexcessive currents and therefore not adequately protect interconnectwiring. As another example, it may be desirable in some applications toprovide an automatic field flash for the generator without the need formanual pilot control.

The need exists, therefore, for a generator controller that canimplement numerous and complex control and monitoring functions tofacilitate proper generator operation without a significant increase incost, space requirements or weight with an increased reliability ofgenerator operation and control.

SUMMARY OF THE INVENTION

In view of the aforementioned needs, the invention contemplates, in oneembodiment, a controller for a starter/generator used with an engine,comprising means for monitoring field free wheeling current in agenerator field winding; means for monitoring generator voltage andcurrent; and microprocessor control means for controlling generatorfield current as a function of said monitored generator current,generator voltage and field current.

The invention further contemplates the methods embodied in the operationof such apparatus, as well as a method for detecting field integrity ina generator comprising the steps of:

a. monitoring field free wheeling current in the generator using arectifier in shunt with the generator field winding; and

b. de-energizing the generator when field free wheeling current isabsent during a time period when field free wheeling current should bepresent.

In another embodiment, the invention contemplates a switching driveroverload protection circuit comprising means for controlling currentthrough the device, and a relaxation oscillator for decreasing saidcurrent in response to increasing load.

These and other aspects and advantages of the present invention will bereadily understood and appreciated by those skilled in the art from thefollowing detailed description of the preferred embodiments with thebest mode contemplated for practicing the invention in view of theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (parts A and B) is a system level functional block diagram of agenerator control circuit according to the present invention;

FIG. 2 is a more detailed schematic diagram of a field integrity monitorcircuit suitable for use with the control circuit of FIG. 1;

FIG. 3 is a detailed schematic diagram of a field integrity signalconditioning circuit used with the field integrity monitor circuit ofFIG. 2;

FIG. 4 is a flow diagram of the microprocessor routine for fieldintegrity check;

FIG. 5 is a schematic diagram of a torque limiting circuit according tothe present invention;

FIG. 6 is a schematic diagram of a field weakening circuit according tothe present invention;

FIG. 7 is a schematic in functional block diagram form of a fail-safevoltage regulator biasing circuit in accordance with the presentinvention;

FIGS. 8 and 9 are flow diagrams of an overvoltage protection routingimplemented in the microcontroller software for the system of FIG. 1;

FIG. 10 is a schematic diagram of a differential ground fault detectioncircuit with built in test capability in accordance with the presentinvention;

FIG. 11 is a functional block diagram of a driver overload protectioncircuit in accordance with the present invention;

FIG. 12 is a detailed schematic diagram of an embodiment of the circuitin FIG. 11;

FIG. 13 is a conventional field flash circuit;

FIG. 14 is a field flash circuit according to the present invention; and

FIG. 15 is a representative graph for certain generator signals duringtorque limiting according to the invention.

FIG. 16 is a representative graph for driver overload protection.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a microprocessor based controller apparatusfor a generator is generally designated with the numeral 10. Althoughthe invention is described herein with particular reference to a DCshunt generator such as can be used conveniently as a starter/generatorfor an aircraft engine, this is for purposes of explanation and shouldnot be construed in a limiting sense. Those skilled in the art willreadily appreciate that the teachings of the present invention can beapplied in AC generators, for example Thus, rather than beingapplication specific, the invention is more broadly construed as beingdirected to monitoring and control apparatus and methods for generators.As used herein, the term “starter/generator” simply refers to the factthat a generator such as described herein can be used as a starter foran engine, and after engine start can be used as a power plant for theaircraft. However, reference to the use of a controller according to theinvention with such a starter/generator is not intended to be limitedonly to generators used as starters.

In accordance with an important aspect of the invention, the generatorcontroller 10 is designed for the use of a microprocessor-basedcontroller or programmable microcontroller 12. The use of amicrocontroller significantly increases the number of monitoring andcontrol functions that can be integrated into the controller 10. Thisincreased flexibility permits monitoring and control functions inaccordance with the invention that heretofore were not used with knowngenerator controllers, and particularly for starter/generators used withaircraft engines. The monitoring and control functions as taught hereinthus facilitate the use of a microcontroller for aircraftstarter/generators, for example, where otherwise the benefits of suchuse of a microcontroller would not be apparent. As will be understoodfrom the detailed description herein, however, some of the teachings ofthe present invention also have general application to generatorcontrollers that are not microprocessor based, although their use with amicroprocessor-based controller enhances their advantages.

With continued reference then to FIG. 1, the controller apparatus 10includes the microcontroller 12 that receives a plurality of inputsignals from various monitoring circuits and issues control signals tothese circuits and other control devices to regulate and controloperation of the generator. The microcontroller can be any suitablemicroprocessor-based device such as part number 87C51 available fromIntel. This device can be programmed in a conventional manner accordingto the manufacturer's specifications to implement the control functionsas described herein and/or set forth in the accompanying flow diagrams,as is well known to those skilled in the art. Peripheral components suchas ROM and RAM memories, clock generators and so on which are inherentlyused in combination with a microprocessor form no particular part of theinvention and thus have been omitted from the drawings for clarity andunderstanding.

Among the monitoring circuits shown in FIG. 1 is a field currentintegrity circuit 14 that provides means for detecting the presence orabsence of a field winding current. The field integrity circuit 14produces an output 16 which in this case indicates the presence orabsence of a field return current. The field return output signal 16 isprovided as an input to the microcontroller for use as will be morefully described hereinafter.

Another circuit that interfaces with the microcontroller 12 is a voltageregulator circuit 18. The voltage regulator circuit 18 is conventionalin design and, in accordance with the invention, can operate independentof the microcontroller 12 in the event that the microcontroller fails.However, in further accordance with the invention, the microcontrolleris programmed to implement a start-up and control function as describedherein via an interface to the regulator circuit 18.

In further accordance with the invention, the microcontroller receivesas an input a digitized representation of the regulated generator outputvoltage, hereinafter referred to as the point of regulation or POR 20.The POR may be connected to the armature through a feeder cable. Asshown at 22 in FIG. 1, the POR analog value is input to a conventionalanalog to digital converter 24 that can receive a plurality of analoginputs 22 in a time multiplexed manner. The digitized POR value is inputto the microcontroller 12. As will be described herein, themicrocontroller 12 uses the POR information in combination with thefield integrity signal 16 to implement an overvoltage protectionfunction that responds more quickly to overvoltage conditions thanheretofore known designs.

Still a further monitoring function of the controller 10 in accordancewith the invention is a ground fault detection circuit 26. The groundfault circuit 26 can be used as a stand alone circuit in controlcircuits that do not use a microcontroller. However, a particularadvantage of the ground fault circuit 26 is that it provides an outputformat that can conveniently be used to implement a built in test thatthe microcontroller can use to verify proper operation of the circuit.

The invention further contemplates the methods embodied in the use ofsuch circuits in combination with the microcontroller, as will beapparent from the descriptions herein.

The generator control unit apparatus 10 further includes a number ofswitching transistors 28, that are used for controlling operation of thegenerator. The numeral 28′ is used to designate the field transistorthat is used to control the field current supplied by the armature in ashunt type generator. These switching devices (the devices 28 and 28′can be the same device or different devices) are susceptible to overloadconditions that can damage and degrade the devices over time or causesudden device failure. In accordance with the invention, these driverdevices are protected with an overload or short circuit protectioncircuit 30. Operation of these circuits are not dependent on the use ofa microcontroller; but, if desired, the microcontroller can be used tomonitor the device output status and de-energize the device asappropriate.

The microcontroller 12 also receives a plurality of digital signals 32from discrete logic circuits. The digital signals are typically input toconditioning circuits 33 of conventional design to transform the signalsinto a format compatible for input to the microcontroller. These digitalinput signals provide information to the microcontroller such as relaystatus that the microcontroller uses to determine if conditions existthat require de-energizing the generator or other control action asappropriate. These digital monitor and control functions form noparticular part of the present invention because they tend to besomewhat design specific and are largely a matter of design choice,other than to the extent that the use of the microcontroller,facilitated by the monitor and control functions of the presentinvention, also conveniently permits these digital signals to be used bythe same control program without increased circuit complexity.

The microcontroller 12 controls overall operation of the generatorthrough a number of relays. For purposes of this embodiment of theinvention, the controller 10 includes a latching field relay designatedKFR in the drawings. The field relay includes a first set of contacts 34(KFR-1) that can be opened by the microcontroller 12 by actuation of arelay trip signal 36. These contacts when open interrupt the shuntcircuit between the generator armature winding 38 and the field winding40, thus protecting the generator and regulator 18 by shutting down thefield current. The field relay includes another set of contacts 42 thatare closed when the KFR is tripped. A non-latching relay designated KSTwith normally closed contacts is used for field flash and is energizedwhen armature voltage is present.

In general terms, the basic functions of the controller 10 include faultdetection for field current, ground fault and generator overvoltageconditions, as well as controlling field current during and after enginestart. These functions are explained in greater detail in the followingdescriptions of the detailed exemplary embodiments of the monitoring andcontrol circuits. As described hereinbefore, these circuits and methodsfacilitate the use of a microcontroller with a starter/generator such asused for aircraft engines.

With reference next to FIGS. 2 and 3, we show an embodiment of a circuit14 for monitoring field integrity. It is noted at this time that thegenerator G is not shown in detail in the drawings because theparticular generator design forms no critical part of the presentinvention. In the described embodiment, the generator is a DC shuntgenerator such as generator 23080-013 available from Lucas AerospacePower Equipment Corporation. However, the field integrity circuit 14 canalso be used conveniently with series and compound generators, forexample.

Generally, a generator will have the field windings disposed on a statorassembly and the armature windings disposed on the armature rotor. In agenerator, when used as a starter, a power source such as a battery (Bin FIG. 1) typically is connected to the armature; and also connected bymeans of a switching device such as a field transistor to the fieldwinding. As a shunt type generator, the field is supplied by thearmature through the switching device.

Thus, as shown in FIG. 2, the controller 10 includes the fieldtransistor 28′ that is gated on and off by a pulse width modulated (PWM)gate control or drive signal 50. The duty cycle of the PWM gate drive 50will be a function of the amount of current required by the field inorder for the controller to regulate the generator output voltage (POR),as well as to regulate the armature current and generator torque. Thecontrol of the PWM gate drive in connection with other functions of thecontroller 10 will be further explained herein.

For purposes of the field integrity circuit 14 operation, themicrocontroller is programmed to strobe the field transistor 28′ offmomentarily during a time period when the gate drive signal normally hasthe transistor turned on. For example, during start-up, the gate controlsignal 50 duty cycle will be 100% (following a torque limiting periodwhich will be explained hereinafter). During such time that the gatedrive 50 is on, the microcontroller 12 cyclically strobes the signal offfor a short period of time, for example, about 1 millisecond. If thefield winding is not open (as in normal operation) a field returncurrent will flow through a free wheeling diode 52 (due to back EMF fromthe inherent field inductance). If the field is open, no return currentwill be detected and the microcontroller can cause the start contactor88 (see FIG. 5) to open, thus terminating the engine start anddisconnecting the generator armature from the battery, therebyprotecting the starter/generator from potential damage due to an openfield winding or connection.

The field integrity monitor function is accomplished in this embodimentwith the use of the free wheeling diode 52, such as part no. 03-0064-30available from Lucas Aerospace Power Equipment Corporation. The freewheeling diode is shunted across the field winding 40. Thisconfiguration is also represented in FIG. 1 wherein the diode 52 isconnected to the field and field return lines 56,58 of the generatorfield. The armature voltage that is supplied to the field through thefield transistor is designated by the numeral 60 in FIG. 2.

With reference to FIG. 3, the field integrity circuit 14 furtherincludes a differential comparator circuit 62 that detects field returncurrent flow through the free wheeling diode 52 by sensing a positivevoltage drop across the diode. The comparator circuit 62 includes adifferential amplifier 64 that is biased with two resistors 66 a,bconnected to a reference voltage 68 (the reference voltage can beobtained in a conventional manner for example from the generator POR).

The output 16 (labelled OPEN SHUNT in FIG. 1) of the amplifier output isnormally logic high due to the presence of a blocking diode 70 and thefact that during operation when the field transistor 28′ is turned on,the free wheeling diode is reversed biased by the field voltage. Whenthe transistor 28′ is strobed off, if the field integrity is good,current flows through the diode 52 causing the amplifier output to go toa logic low state, and the microcontroller detects the comparator output16 during the strobe period to verify the field integrity. If the fieldshunt is open, however, current will not flow through the diode 52, andthe comparator output 16 will not change state during the strobe period.This condition is also detected by the microcontroller 12 during thestrobe period as an indication of the open shunt, and the startcontactor 88 can then be opened.

In addition to checking for an open shunt condition during start-up andsteady-state operation of the generator, this field integrity monitorfunction also will detect generator failures caused by an open fieldreturn, absence of field input power (such as from the armature, forexample) and a failed (open or shorted) field transistor 28′. Forexample, if the field transistor is shorted or latched on, current willnot flow through the free wheeling diode during the strobe period.

FIG. 4 is a flow diagram for a suitable microcontroller 12 routine forthe field integrity monitor function as implemented through theexemplary circuit of FIGS. 2 and 3. From the main processing routine atstep 400, the controller first checks at step 402 whether the generatoris in a start-up mode and a torque limiting feature describedhereinafter has been completed. These conditions correspond to the gatecontrol signal 50 having a 100% duty cycle. After a suitable delayperiod at step 404 from the preceding strobe period, the microcontroller12 strobes the field transistor off at step 406. By choosing a suitabledelay period at step 404, this momentary interruption in field drivedoes not significantly affect the generator starter performance. Thisfield integrity check is performed cyclically throughout the start cycleduring which the field transistor is fully on (PWM is 100%).

A particular advantage of using the free wheeling diode for detectingfield return current as opposed to a current sense resistor, is that thelatter provides a poor signal to noise ratio because the current senseresistance must be small to minimize losses.

Those skilled in the art will appreciate that the field integritymonitor circuit and function can be used independent of otherstarter/generator control functions described herein such as torquelimiting and field weakening.

If field return current is detected, as checked at step 408, then normaloperation is continued. If field return current is not detected, then atstep 410 the start-up sequence is terminated. The field integrity checkis particularly useful during generator start-up to prevent energizingthe armature for a long period of time in an open shunt condition, forexample. However, the field integrity monitor function can also be usedto detect improper generator or control unit 10 (GCU) operation,improper wiring connections and so on.

With reference next to FIG. 5, we show a start-up torque limitingcircuit 77 that can be used conveniently in conjunction with the voltageregulator circuit 18 of FIG. 1. The basic function of the circuit 77 isto utilize a PWM control signal 72, for the field transistor 28′ gatedrive, that has a gradually increasing duty cycle from a selectedminimum, for example about 25%, to about 100%. This PWM control signalcan be produced by the microcontroller 12, for example, and used as anover ride signal 72 (VR INHIBIT in FIG. 1) that is ANDed as at 74 withthe regulator output PWM drive signal 76.

At start-up, under normal conditions the duty cycle of the voltageregulator PWM signal 76 will be 100% in order to provide maximum fieldexcitation current. However, this can cause large starting torque whichmay, for example, stress the engine gearbox. Consequently, the circuitof FIG. 5 is used to limit the starting torque by gradually increasingthe field current by means of gradually increasing the duty cycle of thefield transistor gate control signal.

In the embodiment of FIG. 5, the microcontroller 12 is programmed toproduce a PWM drive signal 78 that has a linearly increasing duty cyclefrom about 25% to about 100%. By ANDing this signal with the regulatorPWM drive signal 76, such as with AND gate 74, the regulator drivesignal is temporarily overridden until the duty cycle of the torqueimiting signal 72 reaches 100%. Thereafter, the voltage regulator PWMsignal can be used to control the field transistor 28′.

Note that in FIG. 5 we show the torque limiting PWM signal 72 asdirectly feeding the field winding through a functional block 84(without showing, for example, the field drive transistor that in actualpractice would be part of the block 84). We show this configurationbecause, from an operational standpoint, that is the basic function andeffect of the torque limiting signal 72, i.e. to control a gradualincrease in the field current. Furthermore, the torque limiting PWMsignal 72 does not have to exhibit a linearly increasing duty cycle, andin fact a significant benefit of the microcontroller 12 is that it canbe programmed to produce a time variant duty cycle of any desired formatdepending on the particular application.

In the circuit of FIG. 5, the armature or starter current is sensed fromthe voltage developed across the interpole winding by a voltage monitor87 which converts the interpole winding voltage to a digital input tothe microcontroller. The torque limiting function is thus de-energizedby means of a software gate 94 until closure of the starter contacts 88(the starter contacts are controlled by the microcontroller in responseto actuation of the manual start switch 90 shown in FIG. 1). When thestarter contacts 88 close, a comparator (realized in software) 92detects the initial starter current and thus enables the gate 94 to passthe torque limiting PWM field control signal 72 through to the fieldtransistor. The circuit 92 and the functions 78 and 94 can be realizedconveniently in the microcontroller 12 software, if so desired.

With reference to FIG. 15, we show a graph of representative responsecurves for starting torque, generator voltage and field current. Notethat the torque limiting function according to the invention causes agradually increasing field current in direct proportion to the generatorvoltage so as to limit the starting torque.

A concept similar to the starting torque limiting circuit 77 of FIG. 5is used to implement a field weakening function. During engine start-up,field weakening is used to lower the field current to reduce starterback EMF to thereby increase armature current and torque. Fieldweakening thus helps maintain starting torque at a desired level.

With reference to FIG. 6 then, a field weakening circuit in accordancewith the present invention is generally designated with the numeral 100.Certain aspects of the circuit 100 are similar to the torque limitingcircuit 77, and in fact those skilled in the art will readily appreciatethat the two circuits can conveniently be implemented together byselective use of logic circuits such as AND gates. Thus, although someof the circuit components for the circuits of FIGS. 1, 5 and 6 can bethe same, we assign different reference designators in the Figuresbecause the individual circuits can be implemented as stand alonecircuits, combined as appropriate or omitted as appropriate based oneach particular application.

The field weakening circuit 100 monitors the armature current bydetecting the voltage drop across the generator interpole winding 86.This voltage (V_(i) in FIGS. 5 and 6) is then converted to a digitalinput for the microcontroller 12 using a conventional analog to digital(A/D) converter device 102 (that corresponds to the monitor circuit 87in FIG. 5. It will be appreciated that the A/D circuit 102 canconveniently be the same as the A/D 24 described with respect to FIG. 1,with the signal INTERPOLE digitally converted in a time multiplexedmanner with the other analog input signals 22 as previously described.

The microcontroller 12 uses the armature current input from theinterpole winding to develop a PWM control signal 104 (γ) that is inputto an AND gate 106. The AND gate 106 can conveniently be the control ANDgate 74 shown in FIG. 1, with the PWM signal γ being part of the PWMvoltage regulator control signal VR INHIBIT.

The PWM field weakening signal γ is ANDed with the output of the voltageregulator 18, which during start-up and normal operating conditions willusually be a PWM signal near 100% duty cycle. Thus, the PWM signal γ canbe used to control the field excitation and thus armature current andtorque according to the programmed field weakening algorithm, which canbe any suitable PWM profile adapted for each particular application. Asshown in FIG. 6, a suitable algorithm includes calculating an error terme=V_(i)−SP, then multiplying the error signal e by a gain factor Av toarrive at duty cycle gamma, where gamma is limited to between 0% and100%. Note that V_(i) is the interpole voltage, SP is a selectedsetpoint and Av is a gain factor. These parameters are applicationspecific and chosen based on generator rating and aircraft enginecharacteristics, as will be understood by those skilled in the art.

Note that the field weakening signal γ (and thus the microcontroller 12)has control of the field transistor 28′ whenever the POR value is belowthe voltage regulator threshold reference 21 (which causes a logic highoutput from the regulator 18). However, when the POR value goes abovethe threshold, the regulator 18 output becomes a PWM signal with a dutycycle based on the regulator control function, and can override the PWMfield weakening signal because of the AND function.

In addition to controlling the field excitation during start-up, as withthe circuit implementations of FIGS. 5 and 6, the microcontroller canalso be used to augment the POR regulation of the voltage regulatorcircuit 18. Such a control function may be desirable in applications,for example, where small adjustments to the POR voltage are useful suchas for load equalization. This added regulation function is accomplishedin the described embodiment using a regulator biasing circuit 110illustrated in FIG. 7 (note that for clarity the implementation of FIG.7 is partially shown in the system diagram of FIG. 1).

An important consideration when using the microcontroller 12 as part ofthe generator output voltage (POR) regulation control is that a failureof the digital control logic such as the microcontroller 12 should notbe allowed to produce an unsafe condition or override basic regulatorfunctions. Therefore, the circuit of FIG. 7 is particularly advantageousin that it provides a fail-safe biasing approach in which if there is afailure in the microcontroller circuit, the voltage regulator 18 cancontinue to operate in a normal manner. In general, this is accomplishedby implementing a zero bias condition from the microcontroller in theevent that the device fails.

In FIG. 7, the microcontroller 12 produces two biasing signals 112 and114 which respectively are bias high and bias low signals. These biassignals are PwM signals, and each signal is passed through a low passfilter 116,118 (which effectively implement the D/A function 510 a inFIG. 1A) to produce corresponding DC high and low bias signals 120,122.The bias low signal 122 in this case is summed with the voltageregulator reference signal 21 by a summation circuit 124, and the biashigh signal 124 is summed with the POR signal by a summation circuit126. Note that the summation for the circuits 124 and/or 126 may includean inversion of the bias signal depending on the particularimplementation of the biasing scheme. In the described embodiment, thebias low signal is subtracted from the reference signal, and the biashigh signal is subtracted from the POR signal, because the POR signal isapplied to the inverting input of the voltage regulator 18.

To implement the fail-safe feature, the bias signals are preferablyunidirectional signals as represented in FIG. 7, such as can beachieved, for example, by the use of open collector switches 128. In theevent of a microcontroller 12 failure, the typical fail mode is suchthat the PWM signals 112,114 will be the same state. In this case thebias from the adjust low signal is canceled by the signal from theadjust high signal by the common mode rejection of the circuit 18, thusremoving any biasing influence of the failed microcontroller 12 from thevoltage regulator circuit 18. Consequently, the voltage regulator 18 cancontinue to regulate the generator output voltage even if themicrocontroller 12 fails.

The generator output voltage (POR) regulation function is, of course, animportant function for the control unit 10. Discrete control systemsused heretofore typically include an overvoltage condition sensingfunction so that if the generator output voltage exceeds a predeterminedthreshold the generator is de-energized. Various specificationsincluding government specifications such as MIL-STD-704 and RTCA-DO-160specify overvoltage protective functions in terms of a voltage/inversetime delay characteristics. This approach limits the time intervalduring which an overvoltage condition is allowed to exist inverselyproportional to its magnitude. In this control scenario, overvoltageconditions are detected for magnitude and time durations and a commonimplementation is to time integrate the voltage transients. A problemwith conventional control schemes is that the discrete circuits must beable to allow a maximum specified transient but prevent a minimum orhigher overvoltage condition. The inherent weakness of this approach isthat during a transient that in actuality is an overvoltage condition(i.e. not an acceptable transient), the overvoltage condition is allowedto increase in magnitude until the control circuit time integrationdetermines that in fact it is an overvoltage and not a transient.

In order to overcome this problem, the microcontroller 12 implements acontrol routine preferably in software that more quickly identifies ordistinguishes an overvoltage condition from a transient. This isrealized by monitoring the regulator 18 status and the overvoltagecondition concurrently. FIGS. 8 and 9 show a microcontroller routine forthe overvoltage protection, with FIG. 8 showing the basic routine andFIG. 9 showing a specific implementation thereof.

Referring to FIG. 8, an overvoltage detection method according to theinvention typically starts from a normal operating mode at step 130. Ifan overvoltage condition is not detected at step 132, normal operationcontinues. If an overvoltage condition is detected, then at step 134 thecontroller 12 checks the on/off status of the voltage regulator. If atstep 136 the regulator is on during the overvoltage condition then thegenerator is shut down at step 138. If the regulator is off during theovervoltage condition, then the overvoltage condition is integrated overtime at step 140 in a conventional manner. If the overvoltage/inversetime test at step 142 passes, then normal operation resumes because theovervoltage condition was a transient. If the test fails at step 142,then de-energizing occurs.

FIG. 9 shows a specific and preferred-implementation of the basiccontrol routine of FIG. 8. In this case, from normal operation at step130, if an overvoltage condition is detected at step 132, a new voltagemeasurement is made at step 144 for purposes of checking whether theovervoltage is increasing, as at step 146. If the overvoltage conditionis not increasing, normal operation is resumed. If the overvoltagecondition is increasing, then at step 148 the microcontroller checks thestatus of the free wheeling diode 52 (keeping in mind that this currentstatus is an input to the microcontroller 12 via the exemplary circuitof FIG. 3). A properly functioning regulator turns off the fieldtransistor meaning that free wheeling diode current should be present.If current is detected in the free wheeling diode at step 150, thennormal operation is resumed. If current is not detected, then thegenerator is de-energized at step 138.

The free wheeling diode current detection previously described isparticularly useful in the overvoltage control routine of FIGS. 8 and 9because the diode return current is an immediate indication that thevoltage regulator for the generator output voltage POR has turned off inresponse to regulator 18 control. If the regulator is still on, then thelikelihood of an overvoltage condition rather than a transient issubstantially higher. However, those skilled in the art will appreciatethat the overvoltage protection function can be realized with othertechniques for detecting the status of the regulator. For example, theon/off status of the field transistor could be detected by themicrocontroller 12.

With reference next to FIG. 10, a differential ground fault detectioncircuit 26 is illustrated. According to an important aspect of theinvention, this circuit includes built in test capability thatfacilitates the use of the microcontroller 12 for ground faultdetection.

The ground fault detection circuit 26 monitors the instantaneousdifference between the generator G return current and the generatorfeeder current. These currents are monitored using a pair of currenttransformers 160,162 which produce a pulse whenever there is a change ofcurrent. The current transformers can each be device no. 50866-000 orGC66 available from Lucas Aerospace Power Equipment Corporation, forexample.

The pulsed outputs of the current transformers are sensed using a pairof differential amplifier circuits 164,166 which are connected tocomplement each other. For example, the return current transformer 162is connected to the inverting input of amplifier 166 and thenon-inverting input of amplifier 164. In a complementary manner, thefeeder current transformer 160 is connected to the inverting input ofthe amplifier 164 and the non-inverting input of the amplifier 166. Inthis manner, the two currents are differentially sensed so that as longas there is no imbalance in the currents, a ground fault is notindicated. The outputs of the amplifiers 164,166 are connected togetherthrough respective diodes 168,170 such that if either amplifier outputgoes low (i.e. negative in the embodiment of FIG. 10), the circuit 26output signal at node 172 is pulled low. If the outputs of bothamplifiers are high, then the output at node 172 of the circuit 26 is alogic high, in this case about S volts. A capacitor 174 is used at theoutput to provide a sample and hold capability for the currenttransformer pulse. This output is monitored by the microcontroller suchas with an A/D converter (see FIG. 1) because the magnitude of the pulsedetected at the sample and hold node is proportional to the magnitude ofthe fault.

Additionally, each of the amplifier outputs 165,167 are connected to aBIT circuit 175 realized with a voltage summing network of resistors 175a,b,c. The output node of the BIT circuit is indicated by the numeral173.

Thus, under normal operating conditions the output of the circuit 26will be between +5 volts and zero (for the embodiment of FIG. 10). Inthis embodiment, the output node 173 of the BIT test circuit is designedto be at a selected value under normal operating conditions of thecircuit 26. This value can be selected, for example, as a midpoint ofthe maximum excursions of the voltage at the BIT node for all circuitfailure modes.

If one or the other amplifier circuits 164,166 fails then the outputvoltage at the BIT node 173 will be biased out of its normal operatinglimits which condition can be detected by the microcontroller 12. Thus,the circuit 26 contains an built in test capability for themicrocontroller to test the circuit 26 based on the output voltage levelat the output node 173 without the use of a test stimulus or signal. Forexample, if the amplifier 164 fails shorted to ground, the voltage atthe node 173 will be about one-half its normal value. As anotherexample, if amplifier 164 fails high, then the voltage at the BIT node173 is twice its normal value.

The output of the circuit 26 is the inverse of the absolute value of thecurrent transformer signals. Differential ground fault sensing inaccordance with the invention provides inherent immunity to common modenoise problems typically found in many environments such as aircraftinstallations. The differential ground fault sensing thus makes thecircuit 26 immune from ground level shifts and common mode noisecoupling that could otherwise produce false readings in conventionalcircuits that rely on absolute ground level comparisons.

As described hereinbefore, the generator control unit 10 can include oneor more power switching driver devices, such as the field transistor28′. With reference to FIGS. 11 and 12, we show an embodiment of adriver overload protection circuit 30 such as can be used with thecontrol unit 10 of FIG. 1. As illustrated in FIG. 11, the protectioncircuit 30 includes the use of a relaxation oscillator 180 which pulsesthe control input of the driver transistor with a duty cycle thatdecreases as output loading increases above a fault threshold determinedby a reference value (194). See FIG. 16.

For purposes of explaining the protection circuit 30, we will assumethat the driver device being protected is the field transistor 28′.However, those skilled in the art will readily appreciate that theprotection circuit 30 can be used in any application for protecting adriver device from an overload condition.

As previously described herein, the field transistor 28′ is controlledby means of a PWM gate control signal, 182 which may, for example,include the starting torque limiting and field weakening controlfeatures of FIGS. 5 and 6, as well as the basic voltage regulator 18control feature. The gate control signal 182 is ANDed with a PWM signal186 from the relaxation oscillator 180, as with AND gate 184. A currentsensing circuit 188 of conventional design is used to sense output loadcurrent in the device 28′. This overload sensing signal is input to acomparator 190 that includes hysteresis and threshold sensing. A delaycircuit 192 delays receipt of the overload signal from the currentsensor. If the overload signal, after the delay, exceeds the referencelevel 194, the comparator trips, turning the device 28′ off regardlessof the drive signal 182 state. As soon as the overload condition dropsbelow the reference level including the hysteresis value, the comparatorresets back again and the gate control signal again takes over controlof the device 28′. The OFF pulse time to the device 28′ from thecomparator 190 will typically be fairly long compared with the dutycycle of the control signal 182 because of the delay function. Thehysteresis and delay times can be selected so that the device 28′operates at a preselected current limited operating point. Under suchconditions, the comparator 190 will pulse at a fairly constant period,defined primarily by the delay period and the hysteresis set points, asthe device 28′ operates near the current limit point. The comparatorwith hysteresis thus operates as a relaxation oscillator. The delayfunction can be used to insure that the relaxation oscillator does notrespond to every minor overload condition, and generally sets the timeconstant of the circuit 30.

FIG. 12 shows a detailed schematic of one embodiment of the circuit ofFIG. 11. In this embodiment, the AND gate 184 is realized by the use ofan optocoupler having a phototransistor 196 that is activated by aphotodiode 198 responsive to the gate control signal 182. The comparator190 with hysteresis can be realized with a conventional circuit designas illustrated in FIG. 12. Note that the output of the comparator 190 atnode 200 provides the collector drive signal for the phototransistor196, thus providing an AND function between the gate control signal 182and the output of the relaxation oscillator. The delay function 192 canbe realized conveniently by the use of an RC time constant establishedby a resistor 202 and capacitor 204.

An amplifier 206 configured as a conventional comparator thus producesthe PWM gate drive signal to the field transistor 28′.

With reference next to FIG. 13, we show a conventional field flashcircuit. As those skilled in the art understand, field flash refers tothe operation in which a generator can be started using the residualvoltage of the generator without the use of an independent voltagesupply to the armature. This residual voltage typically is a minimum of0.5 volts DC for a standard aircraft starter/generator. In theconventional design, a field flash resistor 210 is connected between thefield winding 40 and a contact 212 of a generator control switch GCSthrough a starter relay contacts 216 (KST-1). The KST relay is closedwhen the generator is de-energized and opened when the generator isenergized by the armature voltage. Energizing the KST relay opens thefield flash path. In conventional field flash operation, the GCS switchmust be manually actuated by the pilot even when the field relay KFR isin the reset position as represented in FIG. 13 and only minimalresidual voltage is available. In other words, the field flash path mustbe through the GCS switch.

In accordance with the present invention, as illustrated in FIG. 14, thefield flash resistor is connected between the field relay contacts 214and the field winding 40 so that the KFR contacts are used to select thesource of field flash current. When the KFR is in the trip position(FIG. 14 shows the reset position for the KFR relay, the de-energizedposition for the KST relay and the off position for the GCS), the fieldflash path is selected through the GCS switch as in the conventionalarrangement. When the KFR is in the reset position, the field flash pathis selected to be from the armature. In this manner, field flash canautomatically occur without the pilot having to manually toggle the GCSrelay switch when the KFR is reset which is the normal mode ofoperation.

The invention thus provides a microprocessor based generator controlunit 10 that can control and monitor various generator start-upfunctions and regulation functions with greater responsiveness andcontrol than previously realized.

While the invention has been shown and described with respect tospecific embodiments thereof, this is for the purpose of illustrationrather than limitation, and other variations and modifications of thespecific embodiments herein shown and described will be apparent tothose skilled in the art within the intended spirit and scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. Apparatus for detecting a ground fault in agenerator circuit a load is connected to the generator by a firstconductor, being a feeder line and having a feeder current, and a secondconductor, being a return line having a return current, comprising: asensing means coupled to each conductor for detecting instantaneouschanges in feeder current and return current, where said sensing meansproducing a first output that corresponds to instantaneous changes infeeder current, and a second output that corresponds to instantaneouschanges in return current; a differential circuit for differentiallycombining said first and second outputs and producing a third output forindicating when the ground fault is detected based on a differencebetween said first and second outputs, and; a control means formonitoring said third output and de-energizing the generator when theground fault is detected.
 2. The apparatus of claim 1 wherein saiddifferential circuit differentially combines said first and secondoutputs to reject common mode disturbances that could otherwise cause afalse ground fault indication.
 3. The apparatus of claim 1 wherein saiddifferential circuit comprises a first differential amplifier having anon-inverting input connected to said first output and an invertinginput connected to said second output and producing a first differentialoutput related to difference between said feeder and return currents. 4.The apparatus of claim 3 wherein said circuit comprises a seconddifferential amplifier connected in a complementary manner to said firstdifferential amplifier having a non-inverting input connected to saidsecond output and an inverting input connected to said first output andproducing a second differential output related to the difference betweensaid return and feeder currents.
 5. The apparatus of claim 4 whereinsaid first and second differential outputs are coupled to a common nodeto produce said third output, wherein said third output is related to anabsolute value of the instantaneous difference between said feeder andreturn currents.
 6. The apparatus of claim 1 wherein said third outputindicates magnitude of a ground fault based on the instantaneousdifference between said feeder and return currents.
 7. The apparatus ofclaim 1 wherein said sensing means comprises a first current transformerfor detecting instantaneous changes in the feeder current and producinga pulsed output corresponding to the instantaneous change in the feedercurrent and a second current transformer for detecting instantaneouschanges in the return current.
 8. The apparatus of claim 7 wherein saidtransformers are connected to complementary inputs of said first andsecond differential amplifiers.
 9. The apparatus of claim 7 wherein saidfirst and second current transformers are connected to complementaryinputs of said first and second differential amplifiers to produce saidthird output with said third output being related to the absolute valueof the peak voltage difference between said feeder and return currenttransformer voltage signals.
 10. The apparatus of claim 1 wherein saidcontrol means comprises a built in test (BIT) circuit connected to saiddifferential circuit by which the control means can determine theoperability of said differentially combining circuit independent of theapplication of a test signal.
 11. The apparatus of claim 10 wherein saiddifferential circuit comprises a first differential amplifier having afirst amplifier output that is related to the difference between saidfeeder and return currents, and said BIT circuit comprises a summingnode coupled to said first amplifier output and a reference so that saidnode voltage indicates a failure of the amplifier by a static change inthe voltage level at said node.
 12. The apparatus of claim 11 comprisinga second differential amplifier having a second amplifier output that isrelated to the difference between said first and second conductors'currents, said second amplifier output being coupled to said summingnode so that said node voltage indicates a failure of said first orsecond amplifier by a static change in voltage level at said node. 13.Apparatus for detecting a ground fault in a circuit which has a loadconnected to a D.C. generator by a first conductor being a feeder lineand having a feeder current and a second conductor being a return lineand having a return current, comprising: a first current transformercoupled to said feeder line, said first transformer producing a firstvoltage pulse in response to an instantaneous change in the feedercurrent; a second current transformer coupled to said return line, saidsecond transformer producing a second voltage pulse in response to aninstantaneous change in the return current; a first differentialamplifier having an inverting input and a non-inverting producing anoutput relative to the difference of the voltages at the inverting andnon-inverting inputs; a second differential amplifier, having aninverting input and non-inverting input producing an output relative tothe difference of the voltages at the inverting and non-invertinginputs; means for connecting the inverting input of said firstdifferential amplifier to said first current transformer and receivingsaid first voltage pulse; means for connecting the non-inverting inputof said first differential amplifier to said second current transformerand receiving said second voltage pulse; means for connecting theinverting input of said second differential amplifier to said secondcurrent transformer and receiving said second voltage pulse; means forconnecting the non-inverting input of said second differential amplifierto said first current transformer and receiving said first voltagepulse; a first summing node connected to the output of said firstdifferential amplifier and the output said second differential amplifierwhereby the existence of a ground fault may be detected; a built in test(BIT) circuit connected to the outputs of said differential amplifierswherein the operability of said differential amplifier circuits can betested independent of the application of a test signal, said BIT circuitcomprising a second summing node coupled to said first differentialamplifier output, said second differential amplifier output and areference voltage whereby the second summing node voltage indicates afailure of said first or second differential amplifier by a staticchange in the voltage level at said second summing node; a sample andhold circuit coupled to said first summing node; a control means formonitoring said first and said second summing nodes and de-energizingthe generator when either the ground fault is detected or the failure ofthe apparatus is detected.